1. Field of the Invention
This invention relates to a drive device for a printing head of an ink jet printer in which different size ink droplets are jetted from the same nozzle onto a recording medium such as a recording sheet.
2. Description of the Related Art
An ink jet printer is one type of dot matrix printer. In an ink jet printer, ink droplets are jetted onto the recording sheet according to binary-coded image signals, so that a character or image is formed with recording dots having the same size. In order to form an ink jet image which is variable in gradation, such as a picture, it is essential to reduce the weight of each ink droplet, and therefore the size of the resultant recording dot. If the size of the resultant recording dot is not reduced, the low density region of the printed image will have significant granularity.
A method of decreasing the weight of ink droplets has been disclosed in Japanese Patent Application (OPI) No. 17589/1980. In the method disclosed in this reference, the volume of the pressure chamber is changed, i.e., the chamber is expanded and contracted. In other words, the weight of the ink droplets is reduced by decreasing the force of expanding and contracting the pressure chamber. On the other hand, the high density region of the formed image must be filled completely with ink dots with no spaces between the ink dots. Hence, if the size of each recording dot is decreased, then the printing speed is decreased as much (when compared with the case where the recording dots are large).
Let us consider the cases where print resolving powers are 360 dpi and 720 dpi. In those cases, in order to completely fill the recording sheets with the recording dots, i.e., to form solid prints, the recording dots must be at least 100 .mu.m and 50 .mu.m, respectively. In the case where the print resolving power is 720 dpi, the printing speed is decreased to about a quarter (1/4) of that in the case where the print resolving power is 360 dpi. In order to overcome this difficulty, it is necessary to increase the ink droplet jetting frequency by a factor of "4" (four-fold), or to increase the number of ink jetting nozzles by a factor of "4". However, to do so is rather difficult.
In order to solve the above-described problems, a technique has been proposed in which ink droplets having different weights are jetted from the same nozzle, to form an image that is variable in gradation (cf. Japanese Patent Publication No. 15735/1992, and U.S. Pat. No. 5,285,215). In this technique, a plurality of pulse signals are employed to form a plurality of minute ink droplets, and the number of minute ink droplets thus formed is adjusted to form a larger diameter ink droplet which is dropped onto the recording sheet.
In the technique described above, a plurality of minute ink droplets are combined to form one ink droplet larger in diameter. However, this process decreases the printing speed. Furthermore, since minute ink droplets must be combined before being dropped onto the recording sheet, the recorded dots are unavoidably small in the range of variations in diameter.